The disclosed embodiments of the present invention relate to drive circuitry, and more particularly, to a drive circuitry capable of generating a stable drive voltage beyond a supply voltage.
In order to pass a large current between two nodes (e.g. a source and a drain) of an n-channel metal-oxide-semiconductor field-effect transistor switch (NMOSFET switch or NMOS switch) with a relatively small voltage drop therebetween, a charge pump technique can be used to elevate the gate voltage of the NMOS switch to thereby decrease a turn-on resistance thereof.
In the charge pump technique, charges are dumped onto the gate of the NMOS switch according to alternate phases of a clock signal. By means of a diode or a diode-like device (e.g. a diode-connected MOSFET) which prevents the charges from flowing in the reverse direction, the charges can be stored and accumulated at the gate, thus boosting the gate voltage. However, usage of the diode comes with the penalty of a diode voltage drop which can lower the maximum voltage that the gate of the NMOS switch can be boosted to. This voltage drop can be significant in an on-chip implementation where the supply voltage generally is 5V or lower.
A bootstrap capacitor technique can be more suitable for an on-chip implementation. A bootstrap capacitor is used to boost the gate voltage of the NMOS switch. As charges on the bootstrap capacitor will leak over time, the gate voltage will decrease, possibly to the point where the NMOS switch will turn off. Thus, there is a need for a novel drive circuitry capable of generating a high and stable drive voltage.